Vacuum tube



VACUUM TUBE Original Filed Sept. 29, 1927 lNVENTOR WALTER. VAN B. ROBERTS BY TORNEY Patented July 28, 1936 PATENT OFFICE VACUUM" TUBE Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of-America, a corporation of Delaware Application September 29, 1927, Serial No. 222,696

Renewed September-21, 1934 9 Claims. (erase-ems) This invention relates to the thermionic art and deals'more specifically with a vacuum tube designed to be operated from an alternating current source. It has already been proposed to en- ;l ergize the filament of a three electrode vacuum tube from an alternating current source instead of I from the usual A battery or its equivalent. While an ordinary tube operating as an amplifier, that is, along the portion of the character- 10 istic which is substantially a straight line, may utilize an alternating current filament supply, such use is not fully satisfactory, and when using :the' tube so lighted as a detector, that is,'when the'tube is operating along the portion of' its 16 characteristic curve which is either concave or convex, the hum due to the alternating current source becomes so great as to render this scheme impractical. It has been found that the magf'nitude of that portion of the hum'due to alter- '2 nation of'filament voltage varies as the square of the voltage drop across-the filament of the jtube; "Consequently by reducing the voltage drop across the filament the hum generated can be materiallyreduced. For example, a tube having 25 a filament drop in the neighborhood of 0.8 of a 'volthas been found to give good results.- It is difiicult, however, to produce tubes with so small a filament drop which have a 'sufiicient emission, and moreover, there still remains a cer- 30 tain amount of hum when using these tubes in the ordinary receiver set. 7 My investigation indicates that there are two important factors which govern the hum generated in an alternating current tube. One of these 35 may be called the voltage efiect, and the other may be called the magnetic efiect.

" The voltage effect may be explained as follows: .7

, When an alternating current is flowing in the 40 filament the voltage effective in the plate circuit between the plate andthe centre of the filament is a maximum when the filament voltage is a ,maximum in either direction, and a minimum when the' filament voltage is zero. The varia- 45 tion in plate current which this effect acting by itself would cause, would produce a hum in the output having a frequency twice that of the alternating current source. It is not an effect "of variations of temperature, but of changes in 50 effectivevoltage between plate and filament.

On the other hand, the magnetic effect appears tobe as follows:

With an alternating current flowing in the filamentthe magnetic field which it, produces alters .gss'the paths followed by the electrons, thus causing 'the'tplate" circuit resistance to vary, and from this cause, .the 'plate current varies inversely' as" the "filament current, that is, when the. filament current is a maximum in either direction the plate current is a minimum; and the plate current is a maximum when the filament current is equal to zero. It will thus be 'seenthat the voltage efiect and the magnetic 'efiect exert their influence upon the plate current in opposite phase.

i :1 have found that it is possible to cause these two'effects to substantially balance each other and thus produce a vacuum tube for alternating current-operation which gives a minimum hum in normal use, and my invention consists in the method of and means for accomplishing this result. a i

These and other objects of the invention will bereadily apparent to one skilled in the art from the-following description taken in connection with the accompanying drawing in which:

Fig. 1 is a graphical representation of the variations in plate current corresponding to the variations in filament current;

a Fig; 2 is a graphical representation of the variationsin plate current in accordance with the variations in filament voltage; and

' 'Fig. 3 is a diagrammatic representation of a specific embodiment of my invention.

- Referring in detail to the drawing, the vacuum 'tube'lll is seen to comprise the usual plate and 'gridelements *and I2 respectively. A linear filament l3'suitably designed to have the neces- :Esary'. 1ow voltage drop across its terminals is -:mounted in the tubeLlO. and is connected to the secondary of a transformer M, the primary of .whichmay be connected to any suitable alter- :nating'current supply means. The transformer -l.4iis designed to. reduce the voltage from the zorelativelyf: high value normally available for use, to the low voltage at which the filament is designed to be operated. ,Connected in series with the filament I 3 is a coil l5 which may be wound .either interiorly or exteriorly of the vacuum tube ,andsupported in any convenient manner. This .coil isdesigned to increase the magnetic efiect of the current flowing in the filament. It will be seen th at. in view pi the series connection between the filament and the coil the current in t he coil depends upon the current flowing in the filament, and consequently the magnetic field set up by the coil has an effect in the same phase ,upon the electron stream as the current flowing the filament. v13y properlyproportioning and ,.l ocating thislcoi l l5 itis possible to vary the magnitude of the magnetic field resulting from the current flowing in the filament in such a ments of the tube, or alternatively the number of turns in the coil l5 may be varied to give the desired adjustment.

It has been found in actual practice that a, proper value of the magnetic. field-in the re-Za':

stricted space between plate and filament may be obtained by a proper design of the filament of the p;

vtube, Without resorting to special coils for the like;

while at the same time producing thdesir'ed fila ment temperature and electron emission-. -This;;,

mode of carrying out my invention cannot readily be illustrated by drawings, as it depends upon the proportioning of the resistance of'the filament'so that afilament of] a' given "physical disposition will produce a given magnetic field for a'igiv'en voltage drop. Sinceit is theyoitage variation across. the filament 'whichcauses the voltage effect, while the current variation causes the magnetic effect, it is the resistance which should be a1tered,"(other'things being unaltered) in order to arrive at the desiredmin'imum of fSi'nce the arrangementof the filamentalso determines the eifective field, these several factors all have to be'considered 'in' the correct design of I a tube inaccordance'with this invention. I

1' j The, correct design of a tubein accordance-with the invention is most readily arrived at byethe following process:

' Being iven the grid and plate structure, put

v h in afilament of low voltage say l volt. Connect 40 the tube to some ineans iorobservingthe presence ,oi double frequency current inthe plate circuit. Bringgrid and plate ;returns to,=the centrejof a potentiometer across the filament, Adjustgrid and Pla e potentials o. r n ee erat ne point '45 on the straightest part of the grid potentialplate current characteristic, such point being, known or readily determinable by those skilled in the art. This adjustment makes thewoltaeeefiect ,negligible'but does not eliminate the humefiect due to, the magnetic field of thefilament current. -,NQW. increase.theynegative grid bias gradually --until a minimum offhum or doubleirequency current .inthe plateacircuit (as indicatedby; the aforesaid means) occurs... If the'minimum occurs at substantially the :desired operating bias, the filament structure is satisfa'ctoryand such that no appreciable humoccurs in the plate circuit during normal operation" Howeven if the minimum occurs before the desired operating bias'is reached, the-filament structure is unsatisfactory pinthat the voltage efiect is sufiicient to counterbalance the magnetic effect for that valueof grid bias where the curvature of the grid voltageplate current characteristic is less than prevails at the desired operating bias; In other words under the desired operating conditions the voltage effect predominates: Hence the filament should be replaced by-one having-lower resistance for the samefdimensions, or else by a filament of the satisfactory construction for all around usehas claim is:

' creatinga variable magnetic field sufiiciently inbeen found to consist of the 20 1A tube structure having the resistance of the filament lowered to about 1.5 ohms, the filament'being of the oxide coated type so that sufiicient electronemission is obtained with voltages well under 1.5 volts. metal core of the filament is rectangular in cross s ection and; abdutZ" in length bent in the inverted V shape;

It will readily be understood that the correct relation of internal field and voltage drop may be 10 produced by various specific constructions, and I therefore do not intend to be limited by the de- .tails ofi'thisidisclosure except as defined in the :appended'claims'." 7 a I-Iaving-"thus described my invention,swhat I 10 afc iuriitube having a filamentygrid' and plate, said'filament being adapted to be energized 'fIOmdaiI alternating current source, whereby beingindependfent ofthe filamentandnnder co'n'a troljfof fthe c rrent "flowing in said filament.

'platefsaidffilament being adapted to be energized frornj alternating current source, whereby variation in platecurrentioccur during the nqr mal operation of said tube, and means for magjnetic'ally'generating' variation'sin plate'currentin h opposite phase' tosaidfirstmentioned variations substantially to reduce'said' first mentioned 'yaria-ltionsfsail mean being independent of the filas'aidfilamentf'j v s a 3. 2; yacuum t behaving filamenufgrid nd plate electrodes, s id, filament having a "el" veiy icwwoaage drop assass ns ermmnsfla'riu a" relatively high current, capacity, said tube having it eans; other than the mameav sr creati n agmagnetic field ,sufiicint to ba1ance v riat ons tiflii iefi :jr ine r m the .Yar aim i IQ-l e e th f l me t- Ameuum-tuhehavi g l m n a a i6 intense to produce variationsin plate current,

. saidlastmentioned variations and saidfirst ment on {v ria on ,b ins su tar ti ll q a an lopposfite'imph se- -15. A yacuumz tube havin filament, grid and plate electrodes; said filament having a relatively low voltage drop acrossits terminals-and arelatively high current capacity, said tube having 6.,"A vacuum tube havingafilamentfad pte d to j be energized from a source of variable, potential, [whereby yariationsin jthe plate 'icurrenti'occur during ,the' normal operationv of the" tube,l,sai'd filamentbe'ing providedwithadditional means for {7:0

tense to produce variations in plate'current, said '"last mentioned variations andsaid .first men- 'tion'ed variations being substantially equal: and apposite in phase; said means comprising .ansiri-r The a merit ,andflu'ndercontrolofthe current fiowing'fin be energized iroma source .offvariable potential, 45 w e e a a is s n' em tewr twu due Amsiha q ma 1 Biafi 'n p'iy etim a df -ment being provided, with additional ineans for c n ass mbl ,m r e lsi fiie e t 5 5 ductance coil serially connected with the filament.

'7. A' vacuum tube comprising a plurality of electrodes including a filamentary cathode, and a closely wound coil of wire within the tube envelope coaxially arranged about the cathode and serially connected therewith.

8, A vacuum tube comprising a cathode, a grid electrode surrounding the cathode, an anode sur- 10 rounding the grid electrode and a coil of wire surrounding the anode and electrically connected with the cathode within the tube envelope.

9. A vacuum tube comprising a filamentary cathode, grid and anode electrodes coaxially mounted about the cathode, and a cylindrically shaped coil of wire mounted outside the anode and having as its axis the filamentary cathode, and a connection between the cathode and the coil or wire.

WALTER VAN B. ROBERTS. 

